1. Field of the Invention
The present invention relates to a method and system for charged particle beam lithography and, more particularly, to a method and system adapted for charged particle beam lithography when a circular pattern is lithographically written on a workpiece.
2. Description of Related Art
A variable-shaped electron beam lithography system has been developed and used as a machine that writes a fine pattern at high speed using a charged particle beam (e.g., an electron beam). In this lithography system, a variable-shaped beam of rectangular cross section is formed by the use of two shaping apertured plates each having a rectangular aperture and a shaping deflector interposed between the two apertured plates. The beam is directed at an arbitrary position on a workpiece. The workpiece is irradiated with the beam while varying the shape and area of the cross section of the beam in succession. Consequently, a microscopic pattern can be lithographically delineated at high speed.
In such a variable-shaped electron beam lithography system, it is difficult to write a circular pattern by combining rectangular cross sections of beams. Therefore, a circular aperture apart from the rectangular apertures is formed. Lithography is done using an electron beam of circular cross section formed by the use of the circular aperture.
FIG. 7 schematically illustrates a variable-shaped electron beam lithography system having a circular aperture. An electron beam emitted from an electron gun 1 is directed at a first shaping slit member 4, which is provided with a rectangular aperture 4a. The electron beam of rectangular cross section passed through the aperture 4a is then passed through a shaping lens 6 and directed at a second shaping slit member 7 similarly having a rectangular aperture 7a. The shaping lens 6 brings an image of the aperture 4a into focus at the position of the aperture 7a in the second shaping slit member 7. The shape and cross-sectional area of the beam passed through the aperture 7a in the second shaping slit member 7 can be varied by changing the position of the focused image by means of a shaping deflector 5 located between the first and second shaping slit members 4, 7. The beam passed through the aperture 7a is shot onto a workpiece 13 through an electrostatic lens 9, an objective lens 10, a positioning deflector 11, and a positioning sub-deflector 12.
On the other hand, the second shaping slit member 7 has plural circular apertures A, B, and C of different diameters adjacent to the rectangular aperture 7a. When a circular pattern is written, this circular aperture is used. That is, when a circular pattern is delineated photolithographically, the shaped beam passed through the first shaping slit member 4 is deflected by the shaping deflector 5 such that the beam hits any one of the circular apertures A, B, and C. The beam passed through any one of the circular apertures and given a circular cross section is directed at a desired position on the workpiece 13 while being tightly focused through the electrostatic lens 9, objective lens 10, positioning deflector 11, and positioning sub-deflector 12.
The diameter of each beam of circular cross section can be fundamentally switched between three different values by using circular apertures A (small), B (medium), and C (large) selectively according to the purpose. If the number of circular apertures is increased, the diameter can be switched between a greater number of values. However, the manufacturing costs and space occupied by the slit members impose limitations on the number of the circular apertures.
Accordingly, the incident energy is varied by adjusting the shot time while using plural (e.g., three) shaped electron beams of different diameters. The diameter obtained by development is varied to thereby produce circular patterns of more varied sizes.
FIG. 8 illustrates the relationship between incident energy intensity and the diameter of circular pattern obtained after development. An electron beam 50 of diameter Da is shaped using a circular aperture A, for example. A resist-coated material was irradiated with the beam 50 for three different shot times. The distribution of incident energies that the resist underwent on a line 50D passing through the center of the circular cross section of the beam is shown in FIG. 8. The horizontal axis indicates the position on the workpiece, and O denotes the center of the beam cross section. The vertical axis indicates the incident energy intensity.
Incident energy intensity distribution curve 51 was obtained when the shot time was shortest. Distribution curves 52 and 53 were derived with longer shot times. It can be seen that the incident energy intensity is increased with increasing shot time. L indicates development level of the resist. The distance between the intersections at which the development level intersects with the distribution curves 51, 52, 53 indicates the diameter of the circular pattern obtained by the development. It can be seen from FIG. 8 that as the shot time is increased, the diameter of the circular pattern obtained by development increases from Da+, to Da++, and then to Da+++.
Circular patterns of three different diameters can be obtained by adjusting the shot time between three levels using the electron beam shaped employing the circular aperture A in this way. Circular patterns of nine different diameters in total can be obtained when two additional circular apertures are used if the shot time is varied similarly between three levels.
Where the size is adjusted by adjusting the shot time of the shaped beam (incident energy/electron beam dose) as described above, the following problems take place.
If the size (diameter) is adjusted by the electron beam dose, the variation in size caused when the electron beam dose is changed by a unit amount varies depending on the resist sensitivity. Therefore, when lithography is done on a workpiece, it may not be possible to cover a desired size of circle depending on resist.
This problem is hereinafter described in further detail. FIG. 9 shows an example obtained when the diameter of a circular pattern obtained after development was varied by using three electron beams of different diameters shaped by the use of three circular apertures A, B, and C of different sizes and varying the shot time so as to vary the electron beam dose (see U.S. Pat. No. 8,057,970).
It can be seen from FIG. 9 that the range of diameters capable of being realized by an electron beam shaped using the circular aperture A and the range of diameters capable of being realized by an electron beam shaped using the circular aperture B overlap with each other. Furthermore, it can be seen that the range of diameters capable of being realized by an electron beam shaped using the circular aperture B and the range of diameters capable of being realized by an electron beam shaped using the circular aperture C overlap with each other.
Therefore, it is possible to cope with requirements by adjusting the shot time of an electron beam shaped by using any one of the three circular apertures A, B, and C, from diameter D1 to diameter DN of the required circular patterns.
On the other hand, the same three circular apertures A, B, and C were used. Resist materials different in sensitivity and/or performance from the resist material used in the case of FIG. 9 were used. The diameter of the circular pattern obtained after development varied when the electron beam dose was varied. Variations in the pattern diameter and variations in the beam dose are plotted in FIG. 10. For this resist material, the range of variation of the diameter of the circular pattern obtained after development caused by variation of the incident energy is narrower than for the resist material used in the case of FIG. 10. Consequently, unlike the case of FIG. 9, the diameter of the maximum pattern obtained with the aperture A and the diameter of the minimum pattern obtained with the aperture B do not overlap with each other in the case of FIG. 10. Similarly, the diameter of the maximum pattern obtained with the aperture B and the diameter of the minimum pattern obtained with the aperture C do not overlap with each other. When such nonoverlapping regions BD1 and BD2 exist, it follows that a circular pattern having a diameter lying in any of these ranges cannot be formed.
Especially, resists currently under development are intended to enhance resolution by reducing blurs of the resists themselves. This course of action leads to a smaller decrease in the diameter of the pattern obtained after development when the electron beam dose is changed by a unit amount. Therefore, it can be said that it is becoming increasingly difficult to adjust the size variation by the beam dose.